The present invention relates to a method of controlling a temperature of a molten metal in a ladle or at the outlet of a distributor trough, wherein the thermal energy necessary for this purpose is produced by at least one plasma burner also referred to as a plasma torch.
From European Patent A1-0 180 741 a method and an apparatus for maintaining or increasing the temperature of a molten metal present in a receiving vessel by the feeding of energy are known, wherein the energy necessary for this purpose is introduced by one or more plasma burners. That patent, however, lacks any information as to how the heating power necessary for a predeterminable temperature is to be measured. Furthermore, no limit conditions as to when the temperature to be obtained are mentioned.
In addition, methods are also known in which the energy necessary for the heating is introduced into molten metal by an induction heater that is firmly connected to a distributor trough of a continuous casting installation (see, for instance, European Patent A1-0 132 280). Such methods, however, have the disadvantage, among others, that a separate heating device must be provided for each vessel, which is very expensive and disadvantageous, particularly in the case of teeming ladles.
An object of the invention is to provide a method that gives assurance that the temperature of a molten metal in a ladle or a casting trough at a predeterminable point, for instance at the outlet of the vessel, corresponds to a predeterminable variation with time of the temperature, even if disturbing influences occur, in which connection in the simplest case, the course of the temperature may also be constant (i.e. maintaining the temperature).
The foregoing object is achieved in the case of a ladle in the manner that:
the variation with time of a target temperature, the mass or the mass flow of the outward flowing melt and the specific values of the melt present in the ladle, as well as system parameters, are introduced into an adaptive regulation;
from the foregoing values there is determined a setting signal suitable for the heating power necessary for obtaining the desired course of the temperature;
at the same time, the actual temperature of the melt is continuously measured; and
the setting signal for the heating power, in the event of a deviation of the actual temperature of the melt from its target temperature which exceeds a predeterminable tolerance, is changed by means of a control in the manner such that, in the event that the actual temperature exceeds the target temperature, it is decreased; and in the event that the actual temperature is less than the target temperature, it is increased.
The heating power determined by the regulation, which can in principle also have a course that is a function of the time, can for instance, as shown by experience, produce the desired course of the target temperature in the event that there are no disturbances. However, the feeding back of the measured temperature to a controller serves for adapting the heating power determined on the basis of the values introduced, to the heating power actually necessary for obtaining the target temperature in the event that disturbing influences occur, so that the course of the temperature of the molten metal, for instance at the outlet of the ladle, is adjusted to the target course, except for a predeterminable tolerance.
In connection with a distributor trough in which the melt does not stay but flows through, it is contemplated, in accordance with an aspect of the invention, to introduce into the regulation, in addition to the values to be introduced in the case of a ladle, also (i) the temperature of the melt upon entrance into the distributor trough, (ii) the mass of the melt to be introduced as a whole into the distributor trough and (iii) the mass flow thereof upon the entrance into and the emergence from the distributor trough. In this case, if the actual temperature of the melt is measured at the outlet of the distributor trough and not below the power connection by the plasma burner, the setting signal for the heating power must be changed, with due consideration of the system-determined dead time.
However, it is also possible, in accordance with a further aspect of the invention, to operate the control of the setting signal for the heating power which is necessary in each case without regard to a system-determined dead time if, in addition to the temperature at the outlet of the distributor trough, the temperature in the melt below the plasma burner, and therefore in the region of the introduction of the heat, is also measured. In this case it is even possible to adjust the course of the temperature of the melt at the outlet of the distributor trough with respect to the target course within a predeterminable tolerance even without use of a regulation.
In order to operate the system with favorable efficiency, it is furthermore provided (i) to set the distance corresponding to the arc length between the plasma burner and the melt at a small initial value and to vary the current intensity in accordance with the required heating power, (ii) to compare the heating power required in each case with a heating-power characteristic value that is possible with maximum current intensity and the initial spacing between the plasma burner and the melt, and (iii) to effect the adaptation of the power necessary for the temperature control, as long as the required heating power is below the heating power characteristic value, exclusively via the current intensity (with the arc length equal to the initial spacing), and if the required heating power lies above the heating-power characteristic value, then exclusively via the distance between the plasma burner and the melt (with the arc current equal to the maximum current intensity).